linux-kernel - Re: [PATCH v3 06/14] sched/cpufreq: uclamp: add utilization clamping for RT tasks

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Date:   Tue, 7 Aug 2018 14:54:04 +0100
From:   Quentin Perret <quentin.perret@....com>
To:     Patrick Bellasi <patrick.bellasi@....com>
Cc:     linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Tejun Heo <tj@...nel.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Paul Turner <pjt@...gle.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Todd Kjos <tkjos@...gle.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Steve Muckle <smuckle@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>
Subject: Re: [PATCH v3 06/14] sched/cpufreq: uclamp: add utilization clamping
 for RT tasks

Hi Patrick,

On Monday 06 Aug 2018 at 17:39:38 (+0100), Patrick Bellasi wrote:
> diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> index a7affc729c25..bb25ef66c2d3 100644
> --- a/kernel/sched/cpufreq_schedutil.c
> +++ b/kernel/sched/cpufreq_schedutil.c
> @@ -200,6 +200,7 @@ static unsigned int get_next_freq(struct sugov_policy *sg_policy,
>  static unsigned long sugov_get_util(struct sugov_cpu *sg_cpu)
>  {
>  	struct rq *rq = cpu_rq(sg_cpu->cpu);
> +	unsigned long util_cfs, util_rt;
>  	unsigned long util, irq, max;
>  
>  	sg_cpu->max = max = arch_scale_cpu_capacity(NULL, sg_cpu->cpu);

IIUC, not far below this you should still have something like:

	if (rt_rq_is_runnable(&rq->rt))
		return max;

So you won't reach the actual clamping code at the end of the function
when a RT task is runnable no?

Also, I always had the feeling that the default for RT should be
util_min == 1024, and then users could decide to lower the bar if they
want to. For the specific case of RT, that feels more natural than
applying a max util clamp IMO. What do you think ?

Thanks,
Quentin